1. Field of the Invention
This invention relates generally to reactive plasma sputter deposition processes for forming and depositing insulating films on substrates and more particularly to reactive plasma sputtering processes that utilize an ac power supply connected to at least two anodes driven alternatively negative and positive in the sputtering process.
2. Brief Description of the Prior Art
In traditional dc sputtering processes the target or cathode must be conducting because ions arriving at the target must be able to accept electrons from the target to become neutral gas atoms again. If the target is insulating, its surface would become quickly charged by the arriving ions, which would create a retarding potential which would stop the sputtering process. On the other hand, one can deposit films of insulating material from a metallic target by forming the insulator chemically through reaction with a reactive background gas. This is called reactive sputtering. There is increasing commercial interest in processes involving deposition of such insulating films. New applications of this process include the areas of deposition of wear resistant coatings; insulating films for microcircuits or electronic devices such as capacitors; sophisticated architectural glass coatings; coatings on polyester film for architectural glass laminates or oxygen barriers for food packaging; heat reflecting coatings for high efficiency lamps or induction furnace heat shields; deposition of barrier and functional layers for flat panel displays, including the ITO glass used in LCD displays; and a myriad of other similar functional applications. Added to this are the many reactive PVD processes used to create decorative effects on a wide variety of plastic, natural and artificial fiber, and metal substrates.
A problem occurs at the anode, however, if the reaction product is an electrical insulator. Since the insulating film coats every surface in the chamber, it will surely also coat the anode, whether the anode is a separate element or the chamber walls. The purpose of the anode, of course, is to collect electrons from the plasma to maintain plasma neutrality as ions leave to bombard the target; these electrons form the return current for the power supply. As the anode is coated over with the insulating film, the conduction path for the electrons is also coated over, and the do sputtering process cannot be sustained. This is called the “disappearing anode” problem. In the past, with single target sputtering, a reactive process was run until this effect began to create serious problems, whereupon the system was opened to mechanically scrub off the offending insulating layer from the anode to create a new metallic surface, or to replace the anode with a new, uncoated one. Another drawback related to the coating of the anode with an insulator is that this insulator will generally charge up as the electrons attempt to collect there. This charge can cause an electric field in the insulating film on the anode, which may exceed the dielectric strength of the film material. When this occurs an arc may be formed, and the energy in this arc may cause portions of the film to be ejected from the anode, creating particles, which can become included in the film growing on the substrate. This causes defects which may be unacceptable in the final product. Even if arcing does not occur at the anode, the charging described above can cause electric fields which affect the return of electrons to the anode. This in turn may create a nonuniform plasma density in large systems, causing nonuniform deposition rates along an extended target surface. Any resulting nonuniform film thickness can produce unacceptable coated product from the system (i.e., production yield loss) even before arcing presents a system problem. This charging is not limited to the anode; insulating islands can build up on the target (cathode) surface as well, due to chemical reaction or backscattering redeposition of insulating material outside the sputtering zone of the target. These insulating islands can charge up due to the ion bombardment and create arcing and nonuniform electric fields.
This problem was addressed in U.S. Pat. No. 6,183,605 entitled “AC Powered System for Continuous Deposition of a Cathode Material” issued on Feb. 6, 2001, assigned to the assignee of the instant application and incorporated herein by reference. In this patent, two anodes were provided, each connected to one side of a center tapped ac power source. The center point, or “tap,” of the power supply is connected to a single magnetron sputtering source of the ordinary variety. Once a plasma has been established, when one anode starts positive with respect to the other, the first anode will collect electrons. If it is of sufficient area and without a magnetic field, it will assume a potential close to potential of the plasma. This will cause electron current in one half of the secondary of the transformer, flowing from the first anode to the target. These electrons combine with ions arriving at the target from the plasma to complete the electrical circuit. Meanwhile, the target will be negative with respect to the plasma by half the secondary voltage. This negative potential attracts ions from the plasma to sputter the target. The second anode is driven still further negative with respect to the plasma by the action of the transformer, to twice the target potential. This electrode thus also attracts ions from the plasma, causing sputtering of its surface, which can remove any build-up of insulating materials formed there previously.
Similarly, on the other half-cycle of the power supply, when the voltage is such that the second anode starts positive with respect to the first anode, the second anode will collect electrons, assuming a potential close to the plasma potential. This will cause electron current in the other half of the secondary of the transformer. Again, the target will be negative with respect to the plasma by half the secondary voltage, and will continue to be sputtered by ions from the plasma. On this part of the cycle, however, the first anode is driven negative, causing sputtering of its surface, removing any build-up of insulating materials which might have formed there when it was biased positively. Thus each of the anode elements act alternatively as true anodes (electron collectors) and as sputtered cathodes (ion collectors), depending upon the polarity of the ac power supply.
It is an object of this invention to improve upon the reactive sputtering process of the '605 patent by providing a sputtering system that replaces the transformer with a single center tapped inductor that greatly simplifies the design.
It a further object of this invention to improve upon the reactive sputtering process of the '605 patent by providing a sputtering system that does not rely on a magnetic at all. Such a system will have the advantages of requiring less space and being less costly.